Is there a more efficient way to edit actions alread used in NLA?
The snowflake button is only displayed for actions not added as an NLA strip, or newly created ones. 

For actions already contained in a strip, it's much easier to select the strip and Tab to tweak it. After that, just Tab again to exit strip-tweaking mode. There's no need to replace existing strips: the action's change will be reflected immediately in all strips containing it.



At worst, you will have to Sync Action Length Alt-R on each strips and rearrange them if their action's duration is changed. Replacing action strips is almost never necessary.